A stator body of the general type involved here is disclosed, for example, in the Swiss journal "Bulletin SEV/VSE", volume 68 (1977), No. 14, pages 685-689, and particularly FIGS. 7, 8 and 9.
It has been possible in recent years to achieve a considerable increase in the specific outputs of turbo-generators by increasing the ampere-turns per centimeter in the stator and by simultaneously reducing the stray load losses. A considerable contribution to this was made by measures for reducing the overhang leakage field. The compression plates of the laminated stator body are one component which is affected by the overhang leakage field. In compression plates of the conventional type, considerable stray losses can occur with local concentration and corresponding hot spots. This affects not only the efficiency, but also the operational reliability of the machine and the capability of the machine for being operated under abnormal conditions (for example with increased underexcitation or when running asynchronously). The actual core of the set of problems is found in the abrupt transition between active part and overhang and in the lack of a magnetic insulator or genuine magnetic flux channelling outside the laminated body. These circumstances (particularly the boundary effects and uneven contours and the reaction of induced currents) make any accurate calculation of the spatial end field and its effects more difficult.
It was possible to diffuse the above problems thoroughly by using laminated compression plates. The laminated compression plate is obtained by stacking and bonding stamped magnetic steel sheets to form a ring which obtains a conical outside contour by turning down one side. The laminated stator body is pressed together by means of tension bolts passing axially through it and by means of non-magnetic tiebars which evenly rest against the outside surface of the compression plates. The conical side of the laminated compression plate acts like a continuous tapering of the active laminated body and air gap. As is demonstrated by the field pattern in FIG. 9 of the previously mentioned article in the Bulletin SEV/VSE, a continuous transition of the field densities between air gap and end space is achieved in this manner. In addition, a large proportion of the overhang leakage field finds a defined return path in the artificially extended laminated body. It is particularly the continuous contours at the end of the iron, and the fact that the eddy currents in the laminations have only a weak retroactive effect on the incoming field, resulting in clear reflection conditions, which create ideal prerequisites for simpler and conclusive field analyses.
Due to the favourable introduction of the end field into the laminated compression plate, the remagnetisation losses are uniformly distributed inside the plates and are spatially not much higher than in the active laminated body. As a development compared with conventional designs, therefore, relatively low stray losses and extremely weak heating are produced in this quite exposed part of the machine.
However, the advantageous characteristics, described above, of a laminated compression plate are obtained at the cost of increased expenditure in its production. The individual segments of magnetic steel sheets of the compression plate must not come into metallic contact with each other. However, during the machining of the end face, a burr is frequently produced which can lead to a metallic connection between adjoining segments in the radial or peripheral direction. These bridging points can become heated to an unacceptably high degree during operation and must therefore be removed--which is a tedious and time-consuming activity, especially in the case of the radial shorts.
In addition, due to the conicity of the end face of the compression plate, tie plates must be used which are completely made of non-magnetic material and which are supported on the outer circumference of the compression plate or on the stator housing. Because of the distances to be maintained to the end turns of the winding overhang, these tie plates, which extend to the stator teeth, increase the rod length and thus also the length of the entire machine.